1. Field of the Invention
The present invention relates to improvements in a method for cutting a plurality of silicon monocrystal ingots into wafers, for example, using a wire saw. The present invention further relates to a wire saw cutting method and apparatus wherein, when cutting by bringing the object being cut (i.e. the work) into contact with an array of many strands of wire, the positions of plural pieces of work are shifted in the direction of cutting feed, whereby wire sway is reduced, and wafer thickness uniformity is made highly precise.
2. Description of the Related Art
Wire saw apparatuses are extensively used for cutting multiple pieces of hard and brittle materials such as ceramics, magnetic materials, and semiconductor materials such as silicon monocrystals, and efficiently manufacturing wafers of prescribed thickness in high volume.
To describe an example configuration of a wire saw apparatus used for cutting silicon monocrystal ingots, as diagrammed in FIG. 2A, a wire 4 is deployed so that it is wound parallel, at a constant interval, about the outer circumferences of three horizontally placed long grooved rollers 1, 2, and 3, with the wire 4 fed from a wire bobbin not shown taken up on another wire bobbin after it has traveled around and around the rollers 1, 2, and 3.
In the wire saw apparatus having the configuration described above, ordinarily one silicon monocrystal ingot is cut, but there are times when two or three ingots are cut simultaneously in the interest of productivity. In the case of cutting three monocrystal ingots, for example, these pieces of work 5 are bonded with adhesive materials 7 to work plates 6, and those work plates 6 are mechanically held in a feeder 9 via attachment holding mechanisms 8.
In order to simultaneously cut pluralities of wafers from silicon monocrystal ingots, the ingots, suspended from the feeder 9, are lowered toward the place where segments of the wire 4 are deployed at a certain interval in the axial direction between the upper two rollers 2 and 3 and are traveling in the same direction, and cutting is done by pressing the ingots against the wire 4. There is also a configuration wherein the monocrystal ingots are pressed against the wire while being raised, instead of being lowered.
In order to do cutting with the wire saw of the configuration described in the foregoing, the crystal axis angle was only adjusted in the horizontal direction, turning the silicon monocrystal ingots in the circumferential direction, and lining up the ingots so that the lower edges in the outer circumferential surfaces thereof were positioned in the same horizontal plane H, as diagrammed in FIG. 2A, to perform simultaneous processing on a plurality of ingots.
Because the performance differs between the top and the tail sides after the silicon monocrystal ingots are pulled up, they are cut to prescribed lengths for each specification wanted, and, when making wafers for the purpose of mass production, it is difficult to make the ingots of the same length, whereupon ingots having different lengths must be processed simultaneously.
As diagrammed in FIG. 2B, because monocrystal ingots having mutually different lengths are processed simultaneously, sway occurs in the wire segments passing the ends of those ingots, the paths of the wire segments running over the end surfaces of the ingots are destabilized, in which condition the wire segments cut into other ingots, which constitutes a cause of precision deterioration. In order to keep the wire from contacting the end surfaces of the ingots, moreover, it is necessary to adjust the way the wire is wound every time it is wound, and it is difficult to perform this adjustment in ordinary mass production processes.